Phase asymmetry in synoptic eddy feedbacks on the negatively-skewed winter NAO

Abstract

The winter North Atlantic Oscillation (NAO) exhibits a significantly negative skewness (−0.68) that the stronger dynamic structure characterizes the negative NAO. This study quantitatively investigates the phase asymmetries in different types of synoptic-scale eddy feedbacks associated with the winter-mean NAO, and the roles of different physical processes in eddy feedbacks are further clarified based on previous theories. The integrated feedback induced by eddy vorticity flux and eddy heat flux is stronger in the negative phase of NAO compared to the positive phase, which may provide an atmospheric internal dynamic explanation for the negative skewness of the winter NAO. This phase asymmetry of integrated eddy feedback is majorly contributed by the eddy vorticity flux–induced dynamic feedback, whose related physical processes involving eddy transport and secondary meridional circulation are stronger under the negative NAO. The eddy transport directly generates positive feedback to the NAO flow, and in addition, the secondary meridional circulation modulates the initially vertically-uneven dynamic feedback to become barotropic. In contrast, the weakly positive net contribution of eddy heat flux–induced thermodynamic feedback, which has received less attention in previous research, can somewhat dampen the phase asymmetry of integrated eddy feedback. The eddy heat flux generates positive (negative) feedback on the NAO flow in the lower (upper) troposphere via a secondary meridional circulation, thus leading to the integrated synoptic-scale eddy feedback being relatively stronger (weaker) in the lower (upper) troposphere.